What would North Korea do with South Korea if they captured it and were permitted to keep it?
Within a few years, South Korea would be as poverty-stricken as the north is now and millions of people would be dead.
The north's repressive regime is not designed to encourage production, initiative, entrepreneurs, or anything like that, and if expanded to the south would only lead to mass starvation of two countries instead of just one.
That should be crystal clear comparing the two countries in the 60 years since the war.
As just one example, take a look at this photograph of the region at night:
The North Korean regimes have forfeited any right to rule, let alone to attempt to conquer their neighbors. Their attempts to acquire nuclear weapons and the means to deliver them should be terminated immediately. Didn't we learn this lesson with Hitler?